Coaxial core inductive structures



Nov. 29, 1960 J. STRATTON 2,962,679

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77/25 fitter/ray United States Patent ce 'COAXIAL CORE INDUCTIVESTRUCTURES Jerry L. Stratton, Schenectady, N.Y., assignor to GeneralElectric Company, a corporation of New York Filed July 25, 1955, Ser.No. 524,003

25 Claims. (Cl. 336-83) This invention relates to inductive devices,such as transformers and reactors, and to magnetic core structurestherefore, and more particularly to inductive devices having coaxialmagnetic core structures.

Inductive devices, such as transformers and reactors, are used to modifyeither the voltage or current of an alternating current system;transformers serving to increase or decrease the voltage and reactorsserving to limit the current. A special type of transformer, referred toas a high reactance transformer, serves not only to modify the voltageof the system, but also limits current by virtue of its internalreactance. High reactance transformers are commonly used for startingand operating arc discharge devices such as fluorescent lamps or mercuryvapor lamps.

Inductive devices for use in alternating current systems of relativelylow frequency, i.e., 60 cycles, commonly comp-rise a magnetic corestructure with one or more coils or windings positioned thereon. Thesecore structures conventionally include a portion on which the coils arearranged and an outer or yoke portion surrounding the coils therebyproviding a return path for the magnetic flux. High reactancetransformers may additionally have shunts formed of magnetic materialarranged between the primary and secondary coils for providing a pathfor leakage flux.

In the past, the core structures for inductive devices haveconventionally been formed of a stacked plurality of relatively thinlaminations of magnetic material. These laminations have generally beenpunched from a sheet of magnetic material thus involving a considerablewaste of the material in the punching operation. In addition, the coilsused on the cores of this type have generally had a rectangularconfiguration thus giving the core and coil assembly a somewhatcruciform cross-section; in turn requiring that the core and coilassembly be housed in a case having a rectangular cross-section thusinvolving considerable waste space. Such constructions have thusrequired considerably more material than is theoretically necessary thusadding appreciably to the overall cost of the device.

It is therefore desirable to provide an inductive device having a corestructure which can be formed essentially without waste of material.Such a construction should be adaptable for use in a high reactancetransformer in which instance it should also permit the use of magneticshunts and the series and shunt air gaps well known in the prior art.

An object of this invention is therefore to provide an improvedinductive device incorporating the desirable features set forth above.

Further objects and advantages of my invention will become apparent byreference to the following descrip-. tion and the accompanying drawingand the features of novelty which characterize this invention will bepointed out with particularity in the claims annexed to and forming apart of this specification.

In its broadest aspects, this invention provides new 2,962,679 PatentedNov. 29, 1950 magnetic structures and new coaxial core inductive devicesgenerally comprising central magnetic core portions with one or moreelectrical windings arranged thereon, and magnetic outer yoke coreportions surrounding the windings thereby providing a return path forthe magnetic flux. The coaxial core of these devices is generallyconstructed about an elongated or rod-shaped central core portion and,as compared with prior inductive devices of similar rating, is moreeconomical in materials and thus involves a lower overall cost.

The features of my invention, which I believe to be novel, are set forthwith particularity in the appended claims. My invention, itself,however, both as to its organization and method of operation, togetherwith further objects and advantages thereof may best be understood byreference to the following description and the drawings in which Fig. lis a cross-sectional side elevation view of a low reactance transformeraccording to my invention;

Figs. 2a through 2 show end and partial side elevation view-s of centercoil supporting structures;

Fig. 2k illustrates in plan view a portion of a lanced strip of magneticmaterial used in forming the center coil supporting structure of Figs.21' and 2 Figs. 3a through 3n and 3p show various views of outer yokestructures;

Figs. 4a through 4j show various views of magnetic end structures;

Figs. 5a and 5b are cross-sectional side elevation views and end viewsrespectively of another inductive device of my invention; 7

Figs. 6a and 6b are cross-sectionalside elevation and end viewsrespectively of still another inductive device according to myinvention;

Figs. 7a and 7b are cross-sectional side elevation and end viewsrespectively of an audio transformer made in accordance with myinvention;

Figs. 8a and 8b are cross-sectional side elevation and end viewsrespectively of another audio transformer according to my invention; 7

Figs. 9a through 9e are cross-sectional side elevation views of a highreactance ballast transformer or parts of such a transformer accordingto my invention, and

Fig. 10 shows a coaxial core type solenoid made according to myinvention.

Referring now to the drawing, there is shown in Fig. l a simple coaxialcore low reactance transformer 1 made according to my invention. Thistransformer has a center magnetic core portion 2 of specific structureto be discussed hereinafter. Surrounding the rod-like center coreportion 2 are a primary winding or coil 6 having leads 4 and 5 adaptedto be connected to an external source of alternating current (notshown), and'a secondary winding or coil 3 having leads 7 and 8 adaptedto be connected to a load (not shown). While the windings are shown asconcentrically wound they may be arranged in any desired manneraccording to any particular requirements which are to be met.Surrounding windings 3 and 6 and forming a protective shell therefor isa magnetic yoke portion whose structure will be discussed below.Completing the magnetic circuit between inner core portion 2 and outeryoke portion 9 are mag netic end pieces 10.

It will be realized that the transformer of Fig. l is to be taken asillustrative only of inductive devices which will be discussed herein ofwhich low reactance transformers, high reactance transformer ballasts,solenoids and reactors, among other devices, are typical.

Referring to Figs. 2a through 2k there are shown various configurationsof center magnetic cores which are useful in the practice of myinvention, Fig. 2a and.

d: Fig. 2b are end and partial side views respectively of a center coreportion comprising a plurality of wires or rods 12, of magneticmaterial, such as iron, circular or otherwise, with their axis parallelto that of the core. This construction, which is useful for manypurposes, has low eddy current loss. The stacking factor, that is, therelationship of the quantity of magnetic material in the core to theinternal air gaps, can be increased by using wires or rods of smalldiameters or of nesting shapes. The rods 12 may be insulated one fromthe other as well as held in final shape by a thin coating thereon ofinsulating varnish or resin. Alternatively the oxide film on the rodswhich occurs either naturally or by reason of heat treatment may beutilized for insulation, the rods being held together in a desired coreshape by windings thereon. It will be understood that all core elementsdiscussed hereinafter may be insulated or bound together as describedabove.

Figs. 2c and 2d show end and partial side views of another typicalcenter core portion 13 made up of parallel-arranged relatively thinlaminations 14 of magnetic material such as iron or steel, cut intoproper widths and properly placed to form a core preferably of roundcross-section, though any cross-sectional shape may be used here as inall other magnetic structures described herein. This core again ischaracterized by low eddy current loss. These laminations are cut from astrip of magnetic material thus involving essentially no waste or scrap.

, Core portion 15 shown in end and partial side views in Figs. 2e and 2is a preferred embodiment for a center magnetic core portion. This coreportion comprises a series of relatively thin laminations of magneticmaterial of various widths nested together in quadrants of a circularcross section as shown, although again other shaped cross sections maybe used. The widest laminations 16 are equal in width to the radius ofthe core cross-section and delimit one side of the quadrant while theother delimiting or bounding lamination 17, the thickness of onelamination narrower than lamination 16 is butted against the latterlamination as shown. Successively narrower laminations are nested inperpendicular V- shaped fashion as shown, conforming to the desiredperiphery of the core to form a quadrant of magnetic material which isrepeated for the rest of the core section. Core elements of other shapedcross-sections may be divided into similar sections having apexes at thelongitudinal axis of the core.

Since all of the laminations of the core portion 15 are substantiallydisposed in a radial direction, the eddy current losses are reduced overthat of core portion 13 with its parallel laminations. For the samereason losses due to fringing fiux are substantially reduced.Furthermore, the stacking factor can be maintained at a high level byusing thin laminations and precise nesting.

The core portion 18 shown in end view in Fig. 2g and partial side viewin Fig. 212 is characterized by reduced eddy current loss and fringingflux loss because its laminations 19, 20, 21, 22, etc., all formed ofrelatively thin magnetic material are all disposed radially outwardlyfrom the center of the core. Here the cross-section of the core portionmay be of any shape and is divided into as many sectors or sections asdesired, each sector or section being bounded by laminations 19 equal inwidth to the radius or dimension of the core, the interveninglaminations being of varying widths depending on their thickness and thestacking factor to be obtained. The laminations may also be taperedtoward the core center to permit a higher percentage of magneticmaterial in the core.

Core portion 23 shown in end view in Fig. 2i and partial side view inFig. 2 is very desirable from the point of view of low fringing fluxloss and low eddy current loss. Core portion 23 is made by coilingrelatively thin lanced magnetic sheet material 24 lanced or slit as at25, 26, and 27 of Fig. 2k. These lancings or slits may be as numerous asdesired and in any pattern so long as the sheet is still capable ofbeing physically handled. If desired, the slits may be spread apart andthe edges thereof insulated as with a varnish.

From the point of view of overall usefulness, taking into account theirlosses such as those due to eddy current and fringing fluxes, as well asirrelatively high stacking factors, the center core portion of Figs.2e-2k are preferred.

The center core portions described above are easily adaptable to massproduction. The laminations of Figs. 2a through 2h are readily fed fromspools or other sources through a die which is in the shape desired. Thecore of Figures 21 through 2k may be made by coiling or rolling up sheet24 in any length or width desired and cutting off to size.

Useful outer yoke core structures are depicted in Figs. 3a through 3p.Outer yoke core portion 28 shown in end view in Fig. 3a and in partialside view in Fig. 3b is comprised of a plurality of wires or rods 29 ofmagnetic material, such as iron, circular or otherwise shaped incross-sections which are arranged parallel to the longitudinal axis ofthe core. The elements 29 of the outer yoke can be separated by aninsulating varnish to lower the eddy current loss or the natural oxidefilm thereon may be utilized for this purpose. The core portion may befabricated much as certain of the center core portion by feeding themagnetic stock from reels and through a die over the windings on thecenter cores. As is also the case with the center core portion made ofsimilar elements, the stacking factor may be improved by using wireshapes which nest closely together.

Outer yoke core portion 30 shown in end view in Fig. 3c and partial sideview in Fig. 311 is comprised of a plurality of relatively thinlaminations 31 of magnetic material of like widths, the lengths of whichare parallel to the longitudinal axis of the core portion. The widths oflaminations 31 may be directed radially outward or oriented at anydesired angle to the radius, as shown, for example, in the drawing. Byorienting the laminations at various angles the thickness of the coremay be varied within wide limits for any particular width laminations.Such a core is characterized by low eddy current loss as well as lowloss due to fringing flux. The laminations of this core may beespecially insulated, if desired.

Figs. 32 and 3f show a different outer yoke core portion 32 comprisingcorrugations 33 of magnetic material, the corrugations or folds runningparallel to the longitudinal axis of the core portion. While, for thepurpose of illustration, corrugations or folds 33 are shown spread apartor open, they can be compressed to various degrees to provide a desiredstacking factor or concentration of magnetic material. Since thecorrugations are oriented essentially in radial directions, the eddycurrent loss and the loss due to fringing flux is low. As with coreportion 30 such corrugations may be oriented outwardly to variousdegrees.

The outer yoke core portion 34, an end view of which is shown in Fig. 3gand a partial side view which is illustrated in Fig. 311 is quitesimilar to outer yoke core portion 32 except that the corrugations orfolds 35 are disposed radially. As in the previous structure, thecorrugations may be compressed as desired to obtain various stackingfactors. This core structure may also be coated with an insulatingvarnish as above. Since the sides of the corrugations are radiallyoriented the losses due to eddy current and fringing flux are favorablylow.

The outer yoke core portion 36 shown in end view in Fig. 3i and partialside view in Fig. 3 j consists of a series of annularly orconcentrically arranged laminations 37 of magnetic material having oneor more air gaps 38 to reduce the eddy current and other core losses.Laminations as in the previous case may be especially insulated asdesired. Laminations 37 may also be in the nature aseaet a g of acontinuous coil in which at least one air gap is formed.

Shown in Fig. 3k in end view and in Fig. 31 in partial side view is anouter yoke core portion formed by coiling a lanced sheet of relativelythin magnetic material such as that shown in Fig. 2k with the lancings25, 26, 27, etc., disposed parallel to the longitudinal axis of thecore. It will be realized, of course, that the lancings or slits mayalso be disposed circumferentially, the primary consideration being todivide the core in a radial manner in order to reduce eddy currentandfringing fiux losses. Again the number of lancings may be variedaccording to the particular requirements to be met. Instead of formingcore 39 by coiling a sheet upon itself it may build up of a plurality ofdistinct lanced sheets, with each sheet forming one or more layers ofthe core. From the fabricating point of view, and particularly withrespect to mass production the use of one continuous sheet is preferred.

Figs. 3m, 3n, and 3p illustrate a variation of core 34 of Figs. 3g and3h. This outer yoke core portion 40 is formed of corrugations 41, thefolds in such corrugations being at the ends of the core and the foldscompressed to give any desired stacking factor. A particular feature ofouter yoke core portion 40 resides in the end structure 42 which extendsat right angles to the main body of the core to form a winding andclosing structure which may also be utilized to form a magnetic path toa center core structure. Such an outer yoke core 40 may conveniently bemade out of a punched strip 43 of relatively thin magnetic material asshown in Fig. 3p, which is folded at lines 44, outer yoke core portion40 having its laminations disposed radially and again characterized bylow eddy current and fringing fiux losses. It will be realized that endstructures similar to 42 may be obtained with other core structures suchas those shown in Figs. 3a through 3 by bending the core structures,such as rods 29, or by providing proper end structures in the case oflaminations 31, corrugations 33, etc.

Generally in coaxial core inductive devices of the type described above,some means must be provided for completing the magnetic circuit betweenthe inner or coil supporting portion and the outer yoke portion of thecore. As described above, this may be accomplished by extending the endsof the outer core inwardly to contact the inner core. In many cases itmay be more convenient to provide a separate annular magnetic bridgingpiece or end structure such as is shown in Figs. 4a through 4]. Shown inFigs. 4a and 4b in end and side views respectively is a molded magneticend structure 45 which is comprised of divided magnetic material 36,such as iron powder suspended in a resin 47. Such magnetic material andresin compositions are well known in the art and may be made usingvarious resins and various percentage of the magnetic filler, the resinbinding the magnetic particles together to form a solid structure.Furthermore, such end structures may be molded in place between thecenter and outer cores and firmly bonded thereto so that an improvedmagnetic path is provided. The above end structures can also be formedof suitable sintered material.

Referring to Figs. 4c and 4d, end structure 48 is comprised of aplurality of laminations 49 of relatively thin magnetic materialinsulated from one another by surface oxide or applied insulatingmaterial with their edges oriented in a radial direction to reduce eddycurrent and fringing flux losses. As shown in the case of outer yokecore portion 30 of Fig. 3c, the laminations 49 may also be disposed atvarious angles thus making it possible to obtain end structures ofdifferent radial thicknesses, using laminations of one standard width.

The new end structure 50 of Figs. 4e and 4f is comprised of corrugations51 of relatively thin magnetic material similar to those of Fig. Be, thefolds of said corrugations being disposed parallel to the length of thestructure with the folds disposed generally radially outward. Thecorrugations may be compressed as desired to obtain any particularstacking factor and magnetic material contact. The eddy current andfringing flux losses of this end structure are low.

Shown in Figs. 4g and 4h in end and side views respectively is endstructure 52 which is similar in general construction to outer yoke coreportion 34 in Figs. 3g and 311. Here the corrugations 53 of relativelythin magnetic material have their folds at the ends of the structures,and disposed radially outwardly and may be compressed again to obtainany particular stacking factor. Since the elements of the end structuresare radially oriented the losses due to eddy current and fringing fluxare again low.

The end structure 54 shown in end view in Fig. 4i and side view in Fig.4f is comprised of a plurality of washer type laminations 55 of magneticmaterial each of which has a plurality of radially oriented lancingstherein resulting in a corrugated structure for each washer lamination.These lancings 56 may be varied in number increasing to the point wherethe washer 55 is just capable of being handled physically. It will berealized, of course, that increasing the number of lancings lowers thelosses due to eddy current and fringing flux.

End structures according to this invention may also be fabricated byother means than those shown. For example, an end structure may be madesimilarly to the outer cores shown in Figs. 3a and 3b. The outer yokecore portions shown in Figs. 3a and 3b and 3i through 3! may also beused as end structures simply by reducing their diameter and length. Theend structures made in these manners may also be varied as describedabove in connection with outer cores. In particular end structures,according to Figs. 3k and 31 are characterized by low core losses. Ingeneral, any of the structures shown for a center core portion or an endstructure may be adapted for any of the other two elements dependingupon the characteristics desired and losses which may be tolerated. Thusthe magnetic molded material 47 of Figs. 4a and 4b may be used in centercores and outer cores in many applications. Other adaptations will beapparent to those skilled in the art.

As pointed out above, the new and useful magnetic elements of myinvention may be used in a wide range of inductive devices, either incombination one with the other or in conjunction with known magneticelements, the typical ones of which have been described above, toprovide new and useful combinations.

Referring again to Fig. 1 which shows a coaxial low.- reactancetransformer made according to my invention, for lowest core losses Iprefer to utilize a center core portion of the types shown in Figs. 2athrough 2k. It will be noted that these cores are all laminated in a generally outward or radial direction which reduces eddy current losses aswell as fringing flux losses. In low power applications where the corelosses are not so important a factor as when higher power is used, thecores shown at 11 and 13 are useful with that at 13 generally preferredof these two structures.

The outer yoke core portions shown in Figs. 3a through 311 and 3p areall very useful for general transformer devices, the cores of Figs. 3a,3b, 3i, and 3 being most used where core losses are not of greatimportance. From the point of view of reducing core losses to a minimumI prefer outer yoke core portion shown in Figs. 30, 3e, 3g, and 3k,since here the laminations or elements are all directed edgewise in agenerally outward or radial direction.

Insofar as end structures are concerned I prefer reducing core losses toa minimum with end structures shown in Figs. 4e through 4j. An endstructure patterned after outer yoke core portion 39 of Fig. 3k is alsovery useful.

Here again the outward orientation of the laminations,

7 together with the high obtainable stacking factors dictate thepreference. Where low power applications are to be made an end structureof the molded type shown in Fig. 4a may be used. The core 45 shown herehas the advantage that it may be molded in place between and to thecenter and outer cores thus attaining a tight magnetic circuit.

The various magnetic elements described may be used in any desiredcombination. However, insofar as control transformers which generallyhave a lower power rating and low core loss factor are concerned, Iprefer from the point of view of ease of manufacture and performance, toemploy first a center core portion 13 which is easily made and hassuitably low power core loss along with a combination outer yoke coreportion and end structure 40 or an outer core 35 and an end structure52. Also preferred for control or low power rating transformers is onehaving a center core portion 18, a combination outer yoke core portionand end structure 40, or an outer yoke core portion 35 and an endstructure 52. Another preferred embodiment for such transformers is onehaving a center core portion 13, combination outer yoke core-endstructure 40, or an outer yoke core portion 35 and an end structure 48.Combinations of center core portion 13, end structures 48 and outer yokecore portion 30 or 36 are also useful for small control as Well as audiotransformers. It is to be realized that many other combinations of mynew magnetic structures as well as combinations of old structures withmy new structures may be used for control transformers or those of lowpower rating.

In general, all of the center core portions shown are useful in varyingdegrees as are the outer yoke core portion and end structures, thoughthose utilizing my new structures are preferred and particularly theexamples given above.

For power transformers in which the power rating and core losses arerelatively higher I prefer to use magnetic structures whose structure issuch as to reduce core losses due to eddy currents and fringing flux. Apreferred pover transformer structure based on Fig. 1 has a center coreportion 15, an outer yoke core portion 30, and end structure 52 or acombination outer yoke core portion and end structure 49. Also preferredis a power transformer having a center core portion 18, an outer yokecore portion 30, and an end structure 52 or again a combina tion outeryoke core portion-end structure 40. A third preferred construction forpower transformers is that comprising a center core portion 23, an outeryoke core portion 39, and an end structure 52 or a combination outeryoke core portion and end structure 40. Other combinations of magneticstructures to provide efficient low core loss power transformers willoccur to those skilled in the art. In general cores 15, 18, and 23 arepreferred for the center core portion of power transformers whilepreferred outer yoke core portions are cores 3t), 32. 34, 39, and 40.Preferred end structures are 48, 56, 52, and 54. These structures allhave low core losses which are important in power applications. Themagnetic connection between the magnetic elements may be improved bybinding them at their points of contact with a thin film of resin filledwith divided magnetic material.

Referring now to Figs. a and 512, there is illustrated means forproviding an improved magnetic circuit as desired for inductive devices.Shown is a low reactance transformer 57 having a center core portion 58of the type shown in Figs. 2e and 2 an outer yoke core portion 59similar to that shown in Figs. 3c and 3d and an end structure 60 such asthat of Figs. 4c and 4d, along with primary and secondary windings 61and 62. The elements of transformer 57 are illustrative only and may bevaried as desired in accordance with my invention. As shown, the ends ofcenter core 58 are accurately tapered and the inner periphery of endstructure 60 also tapered 8 to substantially mate with the taper on core58. This construction provides an improved magnetic path of lowreluctance for the transformer which requires a relatively lowmagnetizing current. It is very useful in power distributiontransformers.

Another useful type of low reactance transformer construction is shownin Figs. 6a and 6b which eliminates the need for a separate magnetic endstructure and has only two magnetic joints instead of the usual four.Transformer 63 has a center core portion 64 of wires or rods formed ofmagnetic material, the wires being flared outwardly at the ends of thecore to form a spool to accommodate windings 65 and 66. After the flaredportion 67, the direction of the rods is altered to run parallel to thecenter core portion 64 and in the process spread or flattened out toform an annulus or cylinder 68. While not essential, if the annulus 68is bound together as with a resin or varnish, a non-magnetic end disk69' may be utilized to maintain the shape of the annulus and partiallysupport it. Outer yoke core portion 79 may con veniently be of anyconstruction of my invention and is shown here as similar to that shownin Figs. 3c and 3d. The above construction eliminates magnetic endstructures. Another advantage resides in the manner in which the fluxtransverses the wires at annulus 68. In any center core of magnetic wirethe flux in the central wires must traverse the wires at the end of thecenter core. However, in the structure shown in Figs. 6a and 6b theflaring of the wires and forming of the annulus 68 reduces the depth ofthe core wires or elements and the magnetic path from the center core tothe outer core is lessened and losses reduced. The two magnetic jointsor junctions being at points where the joint area of magnetic materialis relatively large and the flux density therefore low furthercontnibute to a device which has low core loss and low magnetizingcurrent.

The magnetic structures of my invention are also very useful in makingaudio transformers which are typically used as coupling media betweenthe last audio stage and loud speaker in electronic sound systems. Suchtransformers in order to have a wide frequency range should have goodlow frequency response as well as good high frequency response, and besmall in size or economical of material. Low frequency response variesdirectly with the primary inductance which latter resolves itself into amatter of obtaining an optimum air gap length to air gap area ratio. Inconventional E and I core type transformers the area of the air gap isdetermined by the cross-sectional area of the core. However, in acoaxial core type transformer, the air gap length or the distance alongthe core is not determined by the core cross-sectional area but utilizesthe circumference of the core.

As compared with an E and I type core of the same cross-sectional areato obtain an equal air gap area in a coaxial type construction thelength of the air gap need be only one-quarter the diameter of the core.To double the air gap area the air gap length need be lengthened onlyone-quarter of the core diameter. On the other hand, to double the airgap area of an E type core the cross-section must be doubled and hencethe amount of magnetic material doubled.

High frequency response varies indirectly with the leakage reactance,that is the lower the leakage reactance the higher the frequencyresponse. Whereas in E and 1 core transformers the coupling isrelatively poor; that is, any Wire in a primary winding is relativelydistant from a wire similarly situated in a secondary winding, closecoupling and low leakage reactance is obtained in a coaxial typeconstruction because here the primary and secondary windings arerelatively thin or spread out and are concentrically arranged one withthe other.

A novel and useful coaxial core audio type transformer is shown in Figs.7a in cross-sectional side view and 7b in end view. Here transformer 71has a center core portion .72 oflongitudinally disposed wires orrodsformed of magnetic material. Surrounding the core portion 72 are arelatively long and thin primary winding 73 and a secondary winding 74.Surrounding the windings and conforming to their shape and that part ofcenter core portion 72 which extends beyond the windings is outer yokecore portion 75. Outer core portion 75 may typically be made of one ormore coils of a lanced sheet of magnetic material such as shown in Fig.3k with the lancing extending longitudinally. Preferably the core issomewhat thicker than a single sheet and may be of any greater thicknmsdesired. Outer core portion 75 which is longitudinally and radiallysegmented may also be conveniently made up of a plurality of round,square, or otherwise shaped wires of magnetic material which are bent toshape. The outer core is held in place on the center core by cementingit thereto and can also be fixed thereto by collars 76 and other means.

Another embodiment of an audio transformer is shown in Figs. 8a in sidecross-sectional view and in Fig. 8b in end view. Here transformer 77 hasa center core portion 78 of Wires or rods of magnetic material aboutwhich are wound concentrically primary winding 79 and secondary winding80. If desired, a binding collar 81 may be used to hold the center corewires in place at the places where it extends beyond the windings. Outeryoke core portion 82 is comprised in this instance of a punched strip 43of magnetic material as shown in Fig. 3p which is folded along lines 44in accordion fashion as shown in Fig. 321 and placed around the windingsand the center core, the strip having been precut to conform exactly tothe windings and center core. The outer core 82, as in other similarstructures, may be compressed as desired to provide any particularstacking factor. In lieu of a preformed collar 81, resins filled withmagnetic material or other means can be used to hold the center core 78together and bind it to the outer core 82.

While the preceding audio transformers have been described with respectto magnetic structures or elements of certain types, it will be realizedthat various other combinations of my newly disclosed structures as wellas combinations of these structures with old structures may be used.Thus such a transformer of good performance may be made using a centercore portion of the construction shown in Figs. 2c and 2d and aconforming outer yoke core portion of the type shown in Figs. 3m, n, andp or Fig. 7a. It will be understood that all the center core portionsshown herein may be used as may all outer yoke core portions, the outercore portion being easily altered as required to conform to the windingsand center core thus eliminating separate end structures.

The magnetic structures of my invention are also most useful infabricating high reactance ballast transformers for use in starting andoperating arc discharge devices such as fluoroescent and mercury vaporlamps. In operating arc discharge devices, it is desirable to provide arelatively high starting voltage to initiate an electrical discharge inthe lamps. Additionally, all are discharge devices have a negativeresistance characteristic, and current limiting means must be providedto maintain the current therethrough at a non-destructive level. Thesefunctions, that of providing a high starting voltage and after startingof limiting the current to a safe value are provided by the so-calledhigh leakage reactance ballast transformer. In the past such ballasttransformers have conventionally been constructed with a laminated E andI type core having therein loosely coupled primary and secondarywindings with magnetic shunts between the windings for the passage ofthe leakage flux. Air gaps at the magnetic shunts serve to limit thesecondary current to the proper level.

A characteristic of conventional ballast transformers is the extra coreloss due to fringing flux. This loss occurs because an appreciableamount of the fringing flux travegses the laminations of the E and Itype core at id right angles to the laminations. It is highly desirablethat this loss be reduced as much as possible and such a reduction maybe made using the magnetic structures of I this invention. Additionally,whereas in conventional ballast transformers a magnetic shunt must beprovided between the primary and secondary windings for the leakageflux, with a coaxial core type construction I have found that an outerconcentric core completely surrounding the center core so increases thearea of the leakage path between the center and outer cores that amagnetic shunt may not be necessary. Magnetic shunts may however beincorporated in construction in accordance with this invetnion. Inballast transformers made according to my invention the reluctance ofthe leakage path between the center and outer cores is adjusted asdesired by varying the relative diameter of the center core, the insidediameter of the outer core and the length of the cores. Hence, thecurrent may be limited to any desired value.

Shown in Fig. 9a is a side elevation sectional view of a typical highreactance ballast transformer according to my invention. Thistransformer has a center core portion 83 on different parts of which inspaced relationship and thus loosely coupled fashion are wound, aprimary winding 84 with its leads 85 and 86 adapted to be connected toan external source of alternating current part (not shown) and secondarywinding 87 with its leads 88 and 89 adapted to be connected to an arcdischarge device. Separating the primary and secondary winding is a thinnon-magnetic washer 90. Completely surrounding the windings andextending beyond them is a magnetic outer yoke core portion. Between theextending portions of the center and outer cores in order to provide amagnetic path therebetween are magnetic end structures 92.

The center core portion 83 may be any of those set forth in thisapplication. However, I prefer those center core portions shown in Figs.2c through 2 since these are laminated in a generally outward or radialdirection, thus reducing eddy current loss as well as lowering fringingflux losses. Outer yoke core portion 91 can be any of those disclosedherein, but again I prefer those set forth in Figs. 30 through 3/1 andFigs 3k and 3l. The end structure 92 can be of any of the embodimentsshown though I prefer those of Figs. 4c through 4 End caps 93 of anysuitable insulating material may be provided, as well as an outer casing94. Washer may be of any insulating material, such as fiber, resin, andthe like to serve as a barrier between the windings 84 and 87.

Optimum design considerations may require that magnetic shunts be placedbetween the windings of a high reactance ballast transformer such asthat shown in Fig. 9. This is particularly true in lagging currentcircuits where exciting flux and leakage flux are additive in theleakage path in contrast to leading current circuits in which theexciting flux and leakage flux are subtractive in the leakage path. Fig.9b is similar to Fig. 9a except for a magnetic shunt 95 between primarywinding 84 and secondary winding 87. This shunt 95 can conveniently beof any of the structures shown in Figs. 4a through 4 although againthose of Figs. 40 through 41' are preferred. Spacer 96 of non-magneticmaterial is placed as shown to separate shunt 95 from outer yoke 91 andprovide the shunt air gap.

In leading current circuits, that is, in those having a capacitor inseries with the arc discharge device load a series air gap may berequired in the region of the core adjacent the leading currentsecondary winding. Such configurations are shown in Figs. 9c and 9a. InFig. 90

a spacer 96 is placed between end structure 92 and outer In still othercases it. may be desirable to utilize a-- bridged series air gap asdiscussed 'in Patent 2,598,399,

H. W. Lord, assigned to the same assignee as this appli- 11 cation. Hereas shown in Fig. 9e, a portion of the end structure 92 is cut away todefine an air gap 97 suitably located.

I prefer from the point of view of ease of manufacture and low corelosses ballast transformers having a center core portion as shown inFigs. 2e and 2f, and outer yoke core portion as shown in Figs. 3c and 3dand an end structure as shown in Figs. 4c and 4d, 4e and 4f or 4g and4h. Also, preferred are such devices having a center core portion as inthe examples above as shown in Figs. 2e and 2 an outer yoke core portionas shown in Figs. 3g and 3h and end structures as shown in Figs. 4c and4d, 4e, and 4 and 4g, and 4i. In addition to the above preferredembodiments any desired combination of center core portion, outer yokecore portion, and end structures may be used, including those of myinvention as well as all structures in combination with those of myinvention. Using the basic structural features taught herein highreactance ballast transformers having any number of windings may bemade. For example, two-lamp ballast transformers having one primary andtwo secondary windings are readily made incorporating the features setforth.

The magnetic elements or structures described herein may also be appliedto other inductive devices such as solenoids, a typical one being shownin Fig. 10. The solenoid 98 of Fig. has a fixed center core portion 99,end structure 190, outer yoke portion 101 and winding 102 all fixedtogether as shown. Mounted in line with fixed center core 99 and insidethe winding 102 for reciprocating motion therein is a movable centercore 103 having fixed thereto an end structure 104. The variouscomponents of solenoid 98 are constructed similarly to analogous partsof the various devices shown hereinbefore.

The inductive devices and core structures of my invention offer manyadvantages over those of conventional design. Where losses must bereduced to a minimum, my new magnetic core structures can be combined toprovide inductive devices with very low losses. Where such losses arenot of the essence as in low power devices, my structures may be used incombination one with the other or with known structures to provide newand useful devices.

My inductive devices make maximum use of materials. For example, mygenerally circular inner or center core portion as compared torectangular laminated cores of conventional design permit a unit lengthof copper to enclose a greater core area. Neither do my windings requirespecial insultation as at the corners of rectangular coils where dangerof insulation damages is present. The saving in insulation thusoccasioned permits an increased core area.

There is very little or no scrap in the fabrication of my core elementsas compared to conventional E and I type cores. For the same inductanceone may use the same length of winding and use lower permeabilitymagnetic material than with E and I type cores. Likewise, using the samequality magnetic material, the same number of turns requires less wirewith my coaxial core construction.

My magnetic devices are accompanied by reduced noise since all parts areclosely bound together in compact form. My devices are further wellprotected against weathering, etc., the outer core and end piecesprotecting the windings and serving as a shield therefor as well asacting as a magnetic element of the device.

My devices, being relatively long and slender as compared toconventional devices, have a proportionately larger heat dissipatingsurface with a resultant lower operating temperature and longer life.Their compactness also reduces the heat path to the exterior of thedevice and thus enhances its heat transfer characteristic.

.In certain cases as where molded magnetic end structures are utilizedor where the center core portion is 12 expanded at the ends, separatewinding spools and the like are obviated.

The elimination of separate layers of paper msulation between windinglayers except where used purposely to provide Specific reactancecharacteristics represents a saving in cost and bulk.

Inductive devices made according to my invention provide substantialreductions in size and weight. For example, when a conventionaltransformer about twentyfour inches long and twelve inches in diameterwas redesigned in accordance with the invention the size was reduced toa cylinder twelve inches long and six inches in diameter.

High reactance ballast transformer which for compactness of installationare made as small as possible may be substantially reduced in size andweight over conventional E and I core types by utilizing my invention.

As pointed out above, my magnetic structures and devices are furtherwell adapted to continuous mass production and assembly.

While I have described certain specific embodiments of my invention, Iwish to be understood that I desire to protect in the following claimsall changes or modifications thereto which fall within the spirit andscope of those claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A magnetic structure comprising corrugated magnetic material, thecorrugations being compressed together and in the form of an annulus,the folds of said corrugations being parallel to the axis of saidannulus.

2. A magnetic structure comprising a plurality of magnetic laminations,said structure being divided into longitudinally extending sections eachhaving an apex at the longitudinal axis of said structure, said sectionsbeing abutting and respectively bounded and filled with generallyoutwardly extending laminations of such widths as to conform to theshape of said structure.

3. A magnetic structure comprising a plurality of longitudinallyextending sections having apexes at the longitudinal axis of saidstructure, said sections being abutting and respectively bounded andfilled with outwardly extending and inwardly tapered laminations of suchwidths as to conform to the desired periphery of said structure.

4. A magnetic structure having a generally circular cross sectioncomprising a plurality of longitudinally extending sectors, said sectorsbeing abutting and respectively defined by bounding radially extendinglaminations and filled with radially extending laminations of varyingwidths.

5. A magnetic structure having a generally circular cross sectioncomprising a plurality of longitudinally extending sectors, said sectorsbeing defined by radially extending bounding laminations, said sectorsbeing filled with radially extending filler laminations of varyingwidths, said filler laminations and said bounding laminations beingtapered at their inward portions to provide for substantial filling ofsaid sectors.

6. A magnetic structure comprising a plurality of longitudinallyextending radially abutting sections having apexes at the longitudinalaxis of said structure and defined by interlocking bounding laminations,said sections being respectively filled by interlocking fillerlaminations, said filler laminations being respectively shorter thanalternately parallel with one and then the other of said boundinglaminations.

7. A magnetic structure having a circular cross section comprising aplurality of longitudinally extending radially abutting sectors, saidsectors being bound by interlocking bounding laminations forming aV-shape and extending to the periphery of said structure, the sectorsbetween said bounding laminations being filled with interlocking fillerlaminations, said interlocking filler laminations being respectivelyshorter than and alternately parallel with one and then the other ofsaid bounding laminations.

8. An inductive device comprising an integral segmented inner magneticcore member of generally circular cross-section each longitudinalquarter of the core taken in a transverse direction comprising fiatstrips of magnetic material arranged in successively smaller V-sectionsproceeding from, and with the apexes thereof toward, the center of saidcore, at least one winding on said core, said inner core member havingparts respectively extending beyond the ends of said winding, an outeryoke core member of magnetic material at least partially segmented in aradial direction surrounding and having parts respectively extendingbeyond said winding, and segmented magnetic material respectivelypositioned between said extension parts of said inner and outer cores.

9. An inductive device comprising a cylindrical segmented magneticcenter core portion encompassing and including the longitudinal axis ofthe device, at least one winding on said center core, said center corehaving parts respectively extending beyond the ends of said Winding, andat least partially radially segmented magnetic outer yoke core portionsurrounding said winding and having parts respectively extending beyondthe ends thereof, said parts of said outer core extending inwardly atthe ends of said winding and terminating respectively adjacent saidparts of said inner core.

10. An inductive device comprising a segmented magnetic center coreportion, at least one winding on said center core, said center corehaving parts respectively extending beyond the ends of said winding, anat least partially segmented magnetic outer yoke core portionsurrounding said winding and having parts respectively extending beyondthe ends thereof, and divided resin-bonded magnetic materialrespectively positioned between said parts of said center core and saidouter core portions.

11. An elongated inductive device comprising a magnetic center coremember of generally circular cross section, each longitudinal quarter ofthe core comprising separate laminations of magnetic material arrangedin successively smaller V-shaped configurations proceeding from thecenter of said core, at least one winding on said core, said core havingparts respectively extending beyond the ends of said winding, a magneticouter yoke core member surrounding said winding and having partsrespectively extending beyond the ends thereof, said outer corecomprising magnetic laminations arranged in a generally radial edgewisedirection, and annular magnetic structures positioned respectivelybetween said parts of said center and outer cores comprising corrugatedmagnetic material with the folds thereof parallel with the longitudinalaxis of said center core.

12. An inductive device comprising a magnetic center core member ofgenerally circular cross section, each longitudinal quarter of the coretaken in a transverse direction comprising separate laminations ofmagnetic material arranged in successively smaller V-shapedconfigurations proceeding from the axis of said center core, at leastone winding of said center core, said center core having partsrespectively extending beyond said winding, a magnetic outer yoke coremember surrounding said winding and having parts respectively extendingbeyond the ends thereof, said outer core comprising laminations ofmagnetic material arranged in a generally edgewise direction, andmagnetic end structures respectively positioned between said parts ofsaid center and outer cores comprising laminations arranged in agenerally radial direction.

13. An inductive device comprising an integral magnetic center coremember of general y circular cross section, said cross section beingdivided into longitudinally extending sectors, said sectors beingabutting and respectively bounded by radially extending magneticlaminations and filled With radially extending laminations, at least onewinding on said center core, said center core having parts respectivelyextending beyond the ends of said winding, an integral outer yokemagnetic core member surrounding said winding and having parts extendingrespectively beyond the ends thereof, said outer core comprisingmagnetic laminations in a generally radial direction with folds directedin a radial direction, and integral annular magneto structurespositioned respectively between said parts of said center and outercores comprising corrugated magnetic material with folds directed in aradial direction.

14. An inductive device comprising a center core portion of generallycircular cross section comprising a coiled sheet of magnetic material,said sheet having therein longitudinally extending slits, at least onewinding on said center core, said center core having parts respectivelyextending beyond the ends of said winding, an outer yoke core portionsurrounding said Winding and having parts respectively extending beyondthe ends thereof, said outer core comprising magnetic laminationsextending in a generally radial direction, with folds directed in aradial direction, and annular magnetic end structures positionedrespectively between said parts of said center and outer corescomprising corrugated magnetic material with folds extending in a radialdirection.

15. An elongated inductive device comprising a magnetic center coremember of generally circular cross section, each longitudinal quarter ofthe core comprising separate laminations of magnetic material arrangedin successively smaller V-shaped configurations proceeding from thecenter of said core, at least one winding on said core, said center corehaving parts respectively extending beyond the ends of said winding, acorrugated magnetic outer yoke core member surrounding said winding andhaving parts respectively extending beyond the ends thereof with thefolds therein being directed in a radial direction, and annular magneticstructures respectively positioned between the extending portions ofsaid outer and center cores comprising corrugated magnetic materialswith folds therein directed in a radial direction.

16. An inductive device comprising a magnetic center core portion ofgenerally circular cross section comprising a coiled sheet of magneticmaterial having longitudinally extending slits therein, at least onewinding on said core, said center core having parts respectivelyextending beyond the ends of said winding, a magnetic outer yoke coreportion surrounding sa d winding and having parts respectively extendingbeyond the ends thereof, said outer core comprising magnetic laminationsextending in a generally radial direction with folds therein directed ina radial direction, and annular magnetic structures positionedrespectively between said parts of said center and outer corescomprising corrugated magnetic material with folds therein directed in aradial direction.

17. An inductive device comprising a magnetic center core portion ofgenerally circular cross section, said cross section being divided intolongitudinally extending sectors, said sectors being abutting andrespectively bounded by radially extending magnetic laminations andfilled with radially extending magnetic laminations, at least on windingon said center core, said center core having parts respectivelyextending beyond the ends of said winding, a magnetic outer yoke coreportion surrounding said winding and having parts respectively extendingbeyond the ends thereof, said outer core comprising corrrugated magneticmaterial with folds therein directed in a radial direction.

18, An elongated inductive device comprising an integral magnetic centercore member of generally circular cross section, said center core beingcomprised of parallel arranged magnetic laminations, at least onewinding on said center core, said center core having parts respectivelyextending beyond the ends of said winding, a magnetic outer yoke coremember surrounding said winding and having parts respectively extendingbeyond the ends thereof, said outer core comp-rising corrugated magneticmaterial with folds oriented in a radial direction, and annular magneticstructures positioned respectively between said parts of said outer andcenter cores and respectively comprising magnetic laminations orientededgewise in a generally radial direction.

19. An inductive device comprising a magnetic center core portion ofgenerally circular cross section, said cross section being divided intoa plurality of longitudinally extending sectors, said sectors beingabutting and respectively bounded by radially extending laminations andfilled with radially extending laminations, at least one winding on saidcenter core, said center core having parts respectively extending beyondthe ends of said winding, a magnetic outer yoke core portion surroundingsaid winding and having parts respectively extending beyond the endsthereof, said outer core comprising corrugated magnetic material withfolds oriented in a radial direction, and annular magnetic structurespositioned respectively between said parts of said outer and centercores and comprising corrugated magnetic material with folds thereinoriented in a radial direction.

20. An inductive device comprising a magnetic center core portion formedof parallel disposed wires, at least one winding on said center core,said center core having parts respectively extending beyond saidwinding, a magnetic outer yoke coreportion surrounding said winding andhaving parts respectively extending beyond the ends thereof, said outercore conforming to the shape of said winding and said parts of saidcenter core, said outer core comprising segmented magnetic material withthe folds therein directed in a radial direction.

21. An elongated inductive device comprising a magnetic center coremember of generally circular cross section, each longitudinal quarter ofthe core comprising separate laminations of magnetic material arrangedin successively smaller V-shaped configurations proceeding from thecenter of said core, at least one winding on said core, said core havingparts respectively extending beyond the ends of said winding, a magneticouter yoke core member surrounding said winding and having partsextending respectively beyond the ends thereof, said outer corecomprising magnetic laminations arranged in a generally radial edgewisedirection, and annular magnetic structures positioned respectivelybetween said parts of said center and outer cores comprising corrugatedmagnetic material with the folds therein directed in a radial direction.

22. An inductive device comprising a generally circular segmentedmagnetic inner core member, each longitudinal quarter of said inner coretaken in a transverse direction comprising separate laminations ofmagnetic material arranged in successively smaller V-shapedconfigurations proceeding from and with the apexes thereof toward thecenter of said inner core, at least one winding on said core, said innercore having parts respectively extending beyond the ends of saidwinding, a magnetic outer yoke core member at least partially segmentedin a generally radial direction and having parts respectively extendingbeyond said Winding, and magnetic end structure material at leastpartially segmented in a radial direction positioned respectivelybetween said parts of said inner and outer cores, said parts of saidinner core being tapered, said end structure having surfacesrespectively mating with said tapered parts of said inner core.

23. An inductive device comprising a generally circular segmented innermagnetic core member, each longitudinal quarter of said inner core takenin a transverse direction comprising separate laminations of magneticmaterial arranged in successively smaller V-shaped sections proceedingfrom and with the apexes thereof toward the center of said inner core,at least one winding on said inner core, said inner core having partsrespectively extending beyond the ends of said Winding, an outer yokecore member of magnetic material at least partially segmented in aradial direction surrounding said winding and having parts respectivelyextending beyond the ends thereof, and annular magnetic end structuresat least partially segmented in a radial direction positionedrespectively between said parts of said inner and outer core portions.

24. An inductive device comprising an inner core portion formed of aplurality of magnetic rods, the end portions of said rods being bentoutwardly to form banked structures beyond a central winding receivingportion of said core and terminating in flanges parallel to the centralportion of said core, magnetic material between the outwardly bentportions of said inner core, at least one winding on said centralportion of said inner core, and an outer magnetic core at leastpartially radially segmented surrounding said winding and said innermagnetic core.

25. An inductive device comprising a magnetic center core member havingan outer perimeter substantially circular with reference to alongitudinal axis, said center core including a plurality of radiallyextending laminations and generally outwardly extending laminations inabutting relationship interposed between said radially extendinglaminations and disposed so as to occupy the space bounded by said outerperiphery; at least one winding on said core, said core having partsrespectively extending beyond the ends of said winding; a magnetic outeryoke core member surrounding said winding and having parts respectivelyextending beyond the ends thereof, said outer core comprisinglaminations of magnetic material arranged in a generally edgewisedirection; and magnetic end structures respectively positioned betweensaid parts of said center and outer cores and having thin sections ofmagnetic material parallel with the longitudinal axis of said centercore.

References Cited in the file of this patent UNITED STATES PATENTS682,520 Berry Sept. 10, 1901 1,799,011 Fitzsirnmons et a1 Mar. 31, 19311,804,852 Zamboni May 12, 1931 1,883,905 Hartzell Oct. 25, 19322,279,014 Sawyer Apr. 7, 1942 2,498,702 Nahrnan Feb. 28, 1950 2,579,308Dole Dec. 18, 1951 2,696,593 Dole Dec. 7, 1954 FOREIGN PATENTS 710,719Great Britain June 16, 1954

